A BI-RADS 4 lesion under the breast imaging reporting and data system refers to a suspicious abnormality. BI-RADS 4 lesions may not have the characteristic morphology of breast cancer but have a definite probability of being malignant. A biopsy is recommended for these lesions. If possible, the relevant probabilities should be cited so that the patient and her physician can decide on the ultimate course of action. BI-RADS 4 lesions can be further subclassified into: